Oscillating flow combustion chamber



March 29, 1960 A. w. BLACKMAN, JR., ErAL 2,930,195

oscILLATmG FLow coMBusTroN CHAMBER Filed Feb. 14, 1956 FME .R MMF: N NKRm ECE T VAH NLT IEW@ v2p wE. y ww W HH T e A RT AR/ A Y fe B m E rif m yw f -LHM z l 4 M, my 4 w L e ||`w RITSQ l 2,930,195 1C@ Y Patented Malf-29,1960*i osClLLATiNG uLow coMUsTroN CHAMBER Arthur W. Blackman, Jr.,Manchester, and Arthur E.

- Wetherbee, Jr., Newington, Conn., assignors to United AircraftCorporation, East Hartford, Conn., a corporation of Delaware ApplicationFebruary 14, 1956, Serial No. 565,396

Claims. (Cl. Gil-39.69)

This invention relates to combustion chambers and more particularly tocombustion chambers having oscillating flow and combustioncharacteristics.

It is known that highly eflicient burning is att-ained in combustionchambers Whenthe phenomenon known as screech or oscillating liow occurs.However, with this burning characteristic the oscillations are normallyuncontrollable to the extent that in an extremely short time they maybecome suiilciently violent to damage or destroy the particularcombustion chamber. This burning characteristic occurs in afterburnersof turbine type power plants, ramjets, or similar power plants wherehigh temperatures and high burning rates are encountered. This type ofcombustion usually occurs in high output combustion chambers and ischaracterized by high amplitude oscillations occurring at a resonantfrequency of the combustion chamber cavity. When this type of combustionis encountered the heat transfer rate to the cornbustion chamber wall isincreased greatly. This increased heat transfer rate coupled with thehigh stress repetition rates of the pressure oscillations cause rapidfailure of the combustion chamber structural components.

It is therefore an object of this invention to obtain the advantages ofthis type of combustion but to eliminate structural failures bycontrolling the. amplitude of the pressure oscillations.

It is a further object of this invention to increase the fluid mixingassociated with oscillatory combustion and to increase the heat releaserates in the process of the combustion reactions by providing a meansfor completing combustion in a much shorter combustion chamber lengthfor a given efficiency level.

These and other objects of this invention will become readily apparentfrom the following detailedy description of the drawing in which:

Fig. 1 is a schematic sectional illustrationof a combustion chamber ofrectangular cross section constructed according to this invention. A

Fig. 2 is a cross-sectional illustration taken along the line 2-2 ofFig. 1.

Fig. 3 is similar to Fig. 1 but illustrates a combustion chamber havinga circular cross section, and

Fig. 4 is a cross-sectional illustration taken along the line 4--4 ofFig. 3.

Screeching combustion can be used practically if the amplitude of thepressure pulsations are controlled by properly locating the amefrontswith respect to'the combustion chamber walls and by providing a properlydesigned 'absorption liner in the combustion chamber. Although the typeof oscillatory combustion can be further controlled by regulating thefuel air limits, it is the provision of an absorptive liner of propercharacteristics which can best provide highly eiilcient yet safecombustion.

It'is known that the pressure oscillations during screech or oscillatorycombustion can be either acting along the of the ameholder.

axis of ow through the combustion chamber or transversely thereof. Ithas been found that the transverse oscillations produce more etiicientburning and can be produced and controlled in accordance with theteachings of this invention.

Referring to Fig. 1, a combustion chamber is generally indicated at 10including a plurality of fuel nozzles 12. A V type ameholder 14 withsuitable means of ignition 15 is provided with its downstream end.terminating n a lip 16 radially spaced outwardly from the axis of thecombustion chamber. The ignition means 15 may be a pilot ame as shown orany other suitable form such as electrical or pyrotechnic ignition. `Byproperly locating the lip 16 from the axis of thecombustion chamber thepreferred frequency of transverse oscillations during combustion can beobtained. lIn laddition an absorptive liner 20 is provided between thelip 16 and the outer wall 22 of the combustion chamber. The spacing ofthese components is related to other dimensions of the combustionchamber as described hereinafter. It should be noted that theperforations inthe liner 20 com prise radially extending unobstructedpassagesto permit free radial flow of pressure tluctuations.

Fig. 1 illustrates a rectangular combustion chamber the relative shapeand dimensions of Iwhich are more clearly shown in Fig. 2.

Fig. 3 illustrates a circular combustion chamber generally indicated at30 as having fuel nozzles 32 and a conical flame-holder 34 whichterminates in a down-l stream lip 36. The downstream lip 36 is spacedradially from the axis of the flow through the combustion chamber andthis spacing is related to other dimensions of the combustion chamber asdescribed hereinafter. An annular absorptive liner 40 is provided, andthis liner is spaced inwardly from the outer combustion chamber casing42 but is spaced radially outwardly from the lip 36 The particularconfiguration of the circular combustion chamber is better illustratedin Fig. 4.

The particular size and shape of the elements of this inventionnecessary to producethe proper oscillatory combustion with vthe bestefficiency is set forth hereinafter.v Thus for example a transverse modeof oscillation can be excited more easily than a longitudinal mode ofoscillation if the length to diameter ratio of the cornbustion chamberis less thanorequal to approximately 4 (see dimensions on Figs. l and3). When the particular size of the combustion chamber for anyparticular requirement is determined, it is possible to then define thelocation of the flamefront (outer lip of the flameg holder) and the typeof absorptive liner in terms of one or more dimensions of the combustionchamber.

According to this invention, then, it is best to excite the lowestfrequency of a transverse mode of oscillation. This frequency may beexpressed as:

D for cylindrical duct f=2cil for a rectangular duct where f=frequencyC=speed of sound D=liner diameter h=longer side of rectangular ductfrequency (f). Therefore it can be further shown that the linerqualities or design parameter tp is given as cr=percent open area ofabsorber L=air depth behind absorber facing I eff=eifective length ofair column in liner perforations l eff:- [H- 1.770( 1-0.75a')] Y t=linerface thickness 'y=radius of each liner perforation -Hence once thediameter of the combustion chamber is' established (in the case of acylindrical configuration) a. liner lwith the proper design propertiescan be selected.

Thus knowing that design parameter o is equal toi 13.55/D2, which inturn is identical with the variables shown above which comprise o, it ispossible to design or choose the correct type of liner. The same holdstrue for `a rectangular combustion chamber.

It has been found that for etiicicnt operation the distance of theilameholdcr lip from the longitudinal axis of the combustion chamber inthe case of a cylindrical section is approximately equal to a value of0.35 to 0.40 of the diameter ofthe absorptive liner. On the other handthe desirable distance of the ameholder lip from the longitudinal axisin the case o a rectangular section is approximately equal to or greaterthan a value of 0.35 to 0.40 of the dimension of the side of therectangle formed by the duct.

It is readily apparent then that as `a result of this inventioncontrolled oscillations can be produced in combustion chambers to reducespace requirements while at the same time avoiding destructive, highamplitude pressure oscillations.

The term combustion chamber so used herein is intended to includewithout limitation, reaction chambers where chemical reactions,decompositions or other similar process may occur since this inventionis equally applicable thereto.

Although only two embodiments of this invention have been illustratedand described herein, it will become readily apparent that variouschanges and modications may be made in the construction and arrangementof the various parts without departing from the scope of this novelconcept.

We claim:

1. In a combustion chamber, a cylindrical outer casing for defining acylindrical burner duct, said duct having a longitudinal axis, aperforated cylindrical liner coaxially disposed in said duct, said linerwith said duct forming a tuned chamber open at both ends, theperforations of said liner comprising radially extending unobstructedpassages, a conical arneholder in said duct and having an outer liplocated adjacent said liner, said lip being spaced from said axis adistance which is greater than 0.35 to 0.40 of the diameter of saidliner, and means for intro ducing fuel upstream of said liameholder.

2. In a combustion chamber, a cylindrical outer casing for defining acylindrical burner duct, said duct having a longitudinal axis, aperforated cylindrical liner coaxially disposed in said duct, said linerwith said duct forming an annular chamber open at both ends for free iowthrough said chamber, the perforations in said liner comprising radiallyextending unobstructed passages, a conical fiameholder in said duct andhaving an outer lip located adjacent said liner, said lip being spacedfrom said axis a distance which is greater .than 0.35 to 0.40 of thediameter of said liner, the distance from said lip Vto the downstreamend of said combustion chamber being approximately equal-to. a distanceless lthan four times the diameter of said liner, and means forintroducing fuel upstream of said ameholder.

3. In a combustion chamber, a tubular outer casing for dening acylindrical burner duct, said duct having a longitudinal axis, aperforated cylindrical liner coaxially disposed in said duct, said linerwith said duct forming an annular tuned chamber open at both ends alongsaid axis, the perforations in said liner comprising unobstructedpassages running normal to said axis, a flameholder in said duct havinga V cross section and having its downstream end terminating in an outerlip radially displaced from said axis and adjacent said liner, said lipbeing spaced from said axis approximately equal to a distance which isof a value approximately in the range of 0.35 to 0.40 of the diameter ofsaid liner.

4. A combustion chamber having a casing for delining a burner duct, saidduct having a longitudinal axis, means for injecting fuel at an upstreamstation in said duct, imperforate tlameholder means in said ductdownstream of said fuel injecting means, and terminating in at least oneouter lip from which a ilamefront emanates in a downstream direction,said lip being located a distance away from said axis, and an absorptiveliner coaxially disposed with respect to said casing and being radiallylocated between said lip and said casing thereby forming a spacetherebetween, said liner being cylindrical and perforated substantiallythroughout its length, said perforations comprising unobstructed radialpassages extending substantially normal to said axis, the upstream endof` said liner being located adjacent said lip and extending downstreamthereof whereby said liner lies substantially in the amefront andwhereby said space forms a tuned chamber open at its axial ends.

5. A combustion chamber having a casing for deiining la burner duct,said duct having a longitudinal axis, means for injecting fuel at anupstream station in said duct, imperforate flameholder means in saidduct downstream of said fuel injecting means, and terminating in atleast one outer lip from which a amefront emanates in a downstreamdirection, said lip being located a distance away from said axis, and anabsorptive liner coaxially disposed with respect to said casing andbeing radially located between said lip and said casing thereby forminga space therebetween, said liner being cylindrical and perforatedsubstantially throughout its length, said perforations comprisingunobstructed radial passages extending substantially normal to saidaxis, the upstream end of said liner being located adjacent said lip andextending downstream thereof whereby said liner lies substantially inthe amefront and whereby said space forms a tuned chamber open at itsaxial ends, said lip being spaced from said axis a distance which is.035 to .045 of the maximum cross-sectional dimension of said liner, thedistance from said lip to the downstream end ofsaid combustion chamberbeing -approximately equal to a distance less than four times the saidmaximum cross-sectional dimension of said liner.

References Cited in the file of this patent UNITED STATES PATENTS2,517,015 Mock et al. Aug. l, 1950 2,602,292 Buckland et al. July .8,1952 2,658,339 Bonsall Nov. 10, 1953 2,705,401 Allen et al. Apr. 5, 19552,778,192 Kroon Ian. 22, 1957 2,874,536 Benson et al Feb. 24, 1959FOREIGN PATENTS 202,851 Australia July 27, 1956 (Duplicate of U.S.Patent 2,874,536, Feb. 24, 1959) 525,083 Belgium Dec. 31, 1953

